Various conventional proposals have been made in order to improve the impact resistance of thermoplastic resins. For example, it is known that a copolymer containing a diene rubber or an acrylate rubber is incorporated in a vinyl chloride resin (JP-B-39-19035). In addition, for improving impact resistance, a process of increasing the particle size of a rubber component (JP-B-42-22541) and a process for lowering the glass transition temperature (hereinafter referred to as “Tg”) of a rubber component (JP-A-2-1763, JP-A-8-100095) are proposed.
A technique was recently proposed for improving the impact resistance of a thermoplastic resin by using a graft copolymer having a Tg of 0 degree C. or less, containing 0.1 to 5% by weight of a crosslinking agent and containing a hollow rubber having a porosity of 3 to 90% in the form of a latex (WO00/02963).
However, the above-described processes are accompanied by such problems as a marked increase in the raw material cost, and insufficient improvement in a graft copolymer containing a butadiene rubber which has been most popularly used for the improvement of impact resistance.
Specifically, when the amount of the crosslinking agent is too small and becomes less than 0.1% by weight, the rubber particles are collapsed and finely dispersed upon molding or forming. As a result, no stress concentration occurs and the effects of improving impact resistance are not obtained.
Generally, a rubber-containing copolymer such as described above is mixed for improving impact resistance of a thermoplastic resin such as a vinyl chloride resin or the like. The stress concentration of a molded article and generation and expansion of voids in the rubber play an important role. For stress concentration, it is necessary to introduce a rubber component having a modulus much lower than that of the thermoplastic resin. In practice, various rubbers have been introduced and the size or shape of the rubber component has been optimized. It is predicted that generation and expansion of voids in a rubber contribute significantly to the growth of a shear yield to permit a large energy absorption amount upon impact test and would be expected to lead to an improvement of the impact resistance of a rubber-containing thermoplastic resin.
Therefore, it would be very important to learn how to accelerate generation and expansion of voids in the rubber component. Generation and expansion of voids in the rubber component upon impact on a molded article (under stress) depend much on the crosslinked condition of the rubber. When the rubber component is made hollow in advance, expansion of voids would easily proceed under stress.